My U.S. Pat. No. 4,271,119, issued June 2, 1981, describes a potentiometric test device comprising one or more pairs of ion-selective electrodes and an ion bridge in liquid communication with the paired electrodes, FIG. 10. The ion bridge features an internal capillary liquid transport passage formed by two opposed transport surfaces spaced apart a distance effective to induce capillary flow of liquid between the surfaces. Two access apertures are provided to allow both a patient sample and a reference liquid to be introduced into the bridge. Each of the liquids flows into contact with one-half of the paired electrodes on one side of the ion bridge, and into contact with the other liquid at a junction located in a portion of the passage between the two access apertures.
The aforesaid device has proven to be a highly effective device for assaying ionic analytes. There are, however, several aspects which have warranted further improvement.
First of all, the device is intended to be used by simultaneously metering the reference liquid and a patient's body liquid, e.g., serum, at each of the two access apertures, so that the two liquids simultaneously enter the bridge. On occasion, the simultaneous metering does not occur. Unless delay means are provided, such as the gating walls 110 of the device of my aforesaid patent, FIG. 10, flow of one liquid within the bridge occasionally is so rapid (2 seconds or less to flow a distance of about 1 cm) that it reaches the other access aperture before the second liquid is metered. Although walls 110 are effective in providing a delay, there are certain constructions of the bridge which do not readily lend themselves to the incorporation of such gating walls. A reduction in capillary spacing is known to provide a reduced flow rate, but such reduced spacings as fixed dimensions formed by plastic parts, are disadvantageous because (a) they provide only a gradual withdrawal of liquid from the access aperture and (b) narrow spacings are difficult to manufacture within desired tolerances. A gradual withdrawal of the liquid deposited at the liquid access aperture introduces the possibility that, while it is occurring, a portion of the liquid will be splashed out of the access aperture if the test element is inadvertently jarred or bumped.
Second, under extreme conditions, it is possible that the surface tension of the patient sample will be markedly different from that of the reference liquid. If that is the case, it has been found that the liquid with the higher surface tension produces a driving force that can overcome the flow of the other liquid. If the other, lower surface tension liquid is not deposited in the proper sequence relative to the higher surface tension liquid, in some instances the higher surface tension liquid pushes the lower surface tension liquid entirely out of the bridge, up through the access aperture. This causes contamination of the electrode of the lower surface tension liquid, by the higher surface tension liquid.